Allow override default physnets

Some customers want to be able to specify list of physnets
(provider nets) explicitly, and not having default physnet1/2/3.
We already allow override 'bridge_mappings' on computes so
this is logical continuation.

Change-Id: Iff9657c5261b3b2f715a8b1fbd7dd925187a7857
Related-Prod: PROD-21121
5 files changed